1. Field of the Invention
This invention relates to the field of raw sewage treatment. More particularly this invention relates to the field of effective dewatering of sewage sludge by altering its chemical make up following phosphorous removal by lime precipitation in a tertiary sewage treating process. Still more particularly, this invention relates to the field of treating lime dosed sewage sludge with a source of carbon dioxide to facilitate subsequent dewatering thereof by converting the various calcium hydroxide salts and other salts to the carbonate form, and by converting magnesium hydroxide salts which may be present in the sewage to the bicarbonate form.
2. Description of the Prior Art
Tertiary sewage treating procedures and systems are well known and widely used, as is the procedure of dosing of raw sewage with calcium hydroxide to effect removal of phosphorous therefrom. Such dosing is effected in the primary clarifier of a tertiary system. Further phosphorous removal is commonly effected by dosing the activated sludge effluent from a secondary clarifier of such a system, also in known fashion. However, it has become apparent that sludge produced from dosing with lime in the manner described is extremely difficult to dewater, as a result of which subsequent incineration and disposal of the sludge is complicated and rendered difficult as well as expensive.
So far as is known, the procedure disclosed herein for effectively counteracting the difficulties encountered in dewatering sludge produced by dosing sewage at the primary or secondary stages has been unknown heretofore. That is, the improved procedure of introducing a source of carbon dioxide into a tertiary treating sewage system to recarbonate the lime and sewage sludge by transforming the calcium hydroxide and magnesium salts and other salts produced during dosing of the sewage to the carbonate or bicarbonate form has been unknown heretofore. When the salts are thus transformed, subsequent dewatering of the sludge is greatly simplified.
Introduction of carbon dioxide into a sewage treating procedure has been suggested generally in Cardinal and Sherwood U.S. Pat. No. 3,623,975, "Plural Purpose Sludge Incinerating and Treating Apparatus and Method", dated Nov. 30, 1971, assigned to Envirotech Systems, Inc. Also, Davis, U.S. Pat. 3,442,498 dated May 6, 1969 discloses generally the utilization of carbon dioxide in a sewage treatment system prior to formation of any calcium hydroxide in the system and without attendant phosphorous removal or conversion of magnesium hydroxide to the bicarbonate form. However, introduction of carbon dioxide into a lime precipitation tertiary treating procedure for the specific purpose of recarbonating hydroxide salts and other salts in the system to facilitate, simplify and insure effective dewatering of the sludge has not been suggested heretofore so far as is known.